Glitch That Locked Out Investors for Three Hours Is Latest Malfunction in Electronic Trading

A technical glitch knocked out trading in all Nasdaq Stock Market securities for three hours Thursday afternoon, an unprecedented meltdown for a U.S. exchange that paralyzed a broad swath of markets and highlighted the fragility of the financial world's electronic backbone.

Nasdaq officials scrambled to figure out what happened and resume trading. They shared few of their findings with trading firms or the public during regular...